1. Field of the Invention
The present invention relates a voltage regulator (hereinafter referred to as a V/R) capable of achieving an improvement in response property of the V/R and of stably operating with a small output capacitance.
2. Description of the Related Art
According to the conventional V/R, as described in JP 04-195613 A, the V/R is composed of an error amplifier with a single stage voltage amplification. In other words, the conventional V/R has a circuit as shown in FIG. 5. The V/R is composed of: an error amplifier 13 for amplifying a differential voltage between a reference voltage of a reference voltage circuit 10 and a voltage at a connection point of bleeder resistors 11 and 12 that divides an output voltage Vout of the V/R; and an output transistor 14. When an output voltage of the error amplifier 13 is given by Verr, an output voltage of the reference voltage circuit 10 is given by Vref, and the voltage at the connection point of bleeder resistors 11 and 12 is given by Va, if Vref greater than Va is established, Verr becomes lower. On the other hand, if Vrefxe2x89xa6Va is established, Verr becomes higher.
If Verr becomes lower, because the output transistor 14 is a P-ch MOS transistor in this case, a voltage between the gate and the source becomes larger and an ON resistance becomes smaller, with the result that the V/R functions to rise the output voltage Vout. On the other hand, if Verr becomes higher, the V/R functions to increase the ON resistance of the output transistor 14 and to reduce the output voltage, thereby keeping the output voltage Vout at a fixed value.
In the case of the conventional V/R, because the error amplifier 13 is a single stage voltage amplifying circuit, a two-stage voltage amplification structure is obtained by using such a circuit and a voltage amplification stage which is composed of the output transistor 14 and a load 25. A phase compensating capacitor 15 is connected between the output of the error amplifier 13 and the drain of the output transistor 14. A frequency band of the error amplifier 13 is narrowed by a mirror effect, thereby preventing oscillation of the V/R. Consequently, since the frequency band of the entire V/R becomes narrower, the response property of the V/R is deteriorated.
In general, when the response property of the V/R is improved, it is necessary to widen the frequency band of the entire V/R. However, when the frequency band of the entire V/R is widened, it is necessary to increase a consumption current of the voltage amplifying circuit. In particular, when the V/R is used for a battery of a portable device or the like, its operating time becomes shorter.
Also, when a three-stage voltage amplification is used, even if a consumption current is relatively small, the frequency band of the V/R can be widened. However, because a phase is easily delayed by 180 degrees or more, the operation of the V/R becomes unstable, causing oscillation thereof in the worst case. Therefore, in the case of the three-stage voltage amplification, it is required to return the phase at a zero point resulting from the load and an ESR (equivalent series resistance) of the capacitor. Note that, when the ESR is very small as in a ceramic capacitor, in order to reduce a frequency at the zero point, it is necessary to increase a capacitance value of the ceramic capacitor.
In the conventional V/R, in order to ensure the stability against oscillation, it is required to narrow the frequency band. Accordingly, there is a problem in that the response property is deteriorated. In addition, when the response property is improved, the consumption current is increased and the stability is deteriorated, so that a large capacitance is required for the output of the V/R.
Therefore, in order to solve the above-mentioned conventional problems, an object of the present invention is to obtain a V/R which has a preferable response property with a small consumption current and is stably operated with a small output capacitance.
A voltage regulator according to the present invention includes: a reference voltage circuit connected between a power supply and a ground; voltage dividing circuit for dividing an output voltage supplied to an external load, which is composed of a bleeder resistor; and a differential amplifier for comparing an output of the reference voltage circuit with an output of the voltage dividing circuit and outputting a first signal. The voltage regulator further includes: a phase compensating circuit in which a resistor and a capacitor are connected in series; a MOS transistor in which an output of the differential amplifier is inputted to a gate electrode, which is connected between the power supply and the phase compensating circuit, and in which a source is grounded; a constant current circuit connected between the MOS transistor and the ground; and an output transistor in which a second signal from a connection point between the MOS transistor and the phase compensating circuit is inputted to a gate electrode and which is connected between the power supply and the voltage dividing circuit. Further, a resistor side of the phase compensating circuit is connected with an output terminal of the differential amplifier and a capacitor side of the phase compensating circuit is connected with a drain electrode of the MOS transistor. In addition, the output voltage is outputted from a connection point between the output transistor and the voltage dividing circuit.
The voltage regulator according to the present invention is characterized in that a value of the capacitor is equal to or larger than a gate capacitance value of the output transistor.
The voltage regulator according to the present invention is characterized in that a value of the resistor is equal to or larger than 20 kxcexa9 and the value of the capacitor is equal to or larger than 10 pF.